1. Field of the Invention:
The present invention relates to a hydraulic twin-tube type shock absorber with a leaf valve and a washer plate which are coupled to a base valve by caulking a valve guide and more specifically relates to a structure for preventing the leaf valve from being permanently deformed when the valve guide is caulked during construction of the shock absorber.
2. Description of the Prior Art:
A hydraulic shock absorber composes one member of a suspension system for a vehicle and functions to damp bound/rebound motion of the vehicle. A conventional twin-tube type shock absorber comprises an outer shell and a cylinder disposed within the outer shell. A space defined between the outer shell and the cylinder is filled with gas and oil and constitutes a reservoir chamber. The reservoir chamber is separated from a space inside of the cylinder by a base valve which is fixed to a lower end portion of the cylinder. The base valve has a damping port and a return port, and oil flowing through the damping port generates a damping force under a compression stroke of the shock absorber. A piston which is slidably inserted in the cylinder divides the space inside the cylinder into two chambers. The piston has a damping port and a return port, and the oil flowing between the two chambers through the damping port generates a damping force under a tension stroke of the shock absorber. A piston-rod which is fixed to the piston penetrates an upper end plate of the shock absorber and an upper end portion of the piston-rod is connected to a member of the vehicle.
As shown in FIG. 5, the structure of the base valve portion of the conventional shock absorber comprises a base valve 100, an annular leaf valve 102 for varying an opening area of the damping port by being elastically deformed, a leaf valve seat 118 provided on the lower side of an inner portion of the leaf valve 102, an annular washer plate 116 provided on the lower side of the leaf valve seat 118, and a valve guide 110 having a rod portion which penetrates the base valve 100, the leaf valve 102, the leaf valve seat 118 and the washer plate 116. The valve guide 110 has a flange portion at an upper end portion and a caulked portion 112 at a lower end portion. The caulked portion 112 is formed after the base valve 100, the leaf valve 102, the leaf valve seat 118 and the washer plate 116 have been mounted onto the rod portion, and these members are squeezed together between the flange portion and the caulked portion 112. For the purpose of preventing mutual rotation between these members around their axis, the caulked portion 112 must be caulked sufficiently to communicate to the members a sufficient squeezing force. When the valve guide 110 is sufficiently caulked, the washer plate 116 receives a large axial force from the caulked portion 112 and especially the radially inner portion of the washer plate 116 receives an extremely large axial force from the rounded portion which is naturally formed between the upper surface of the caulked portion 112 and the outside surface of the rod portion of the valve guide 110.
The base valve 100 is usually formed of a sintered material. In case the base valve 100 is constructed of a sintered material, upper and lower corners of the center hole of the base valve 100 is likely to be fractured when the center hole is perforated by a press. Therefore, step-like recesses 108 and 106 which recede from the upper surface 104 and the lower surface 102 of the base valve 100, respectively, are preferably formed at the upper and lower corners of the center hole of the base valve 100, though such recesses 108 and 106 should not be included in a prior art. In case such recesses 108 and 106 are provided in the base valve 100, the radially inner portion of the washer plate 116 will be pushed and deformed toward the step-like recess 106 and as a result, the radially inner portion of the leaf valve seat and the radially inner portion of the leaf valve 118 will be permanently deformed into the step-like recess 106 when the lower end portion of the valve guide 110 is caulked. Such permanent deformation the leaf valve 118 will deteriorate a smooth contact of the leaf valve 118 with the seat surface 114 of the base valve 100 and stable damping characteristics under a compression stroke will not be obtained. Further, such permanent deformation of the leaf valve 118 which is usually constructed of a laminated member of thin plates will cause gaps 120 between the thin plates, that is, mutual separation of the thin plates and may destroy the leaf valve 118.